Traditional systems for the orthodontic movement of teeth have typically consisted of the use of a metal wire that is deformed and bent into a shape so as to provide a load on an orthodontic bracket attached to the tooth so as to move it in a predetermined direction. These metal orthodontic archwires are generally formed of a stainless steel, Co--Cr, or titanium based alloys. Two serious disadvantages with such prior art orthodontic archwires is that they have relatively low shape recovery and the force applied by the wire varies substantially as the tooth moves thus requiring frequent adjustment or replacement by the orthodontist. In order to overcome the disadvantage of such wires, it has been suggested in the prior art the use of a shape memory superelastic alloy material for orthodontic archwires. The advantage of these shape memory superelastic alloy orthodontic archwires is that they are able to apply a substantially constant load during movement of the tooth, thus improving efficiency of the orthodontic procedure. Typically, these superelastic alloys are made of a Ni-Ti alloys. An example of such orthodontic archwires are discussed in U.S. Pat. Nos. 4,037,324 and 4,490,112. A limitation encountered with such prior art-type Ni-Ti alloys archwires is that the amount of force applied by the orthodontic archwire to the orthodontic bracket is relatively low thus requiring longer treatment time. An additional problem encountered with such wires is that initial force necessary to engage the wire with the orthodontic bracket is quite high, thus making it difficult for the orthodontist to apply the archwire to the bracket. A further problem encountered with prior art orthodontic superelastic archwires is the substantially constant load is effective for only a relatively short distance and at a relatively low level of force.
Applicants have discovered that by controlling the composition of the shape memory alloy, the disadvantages of prior art shape memory archwires can be minimized or eliminated.